資料結構對齊, #pragma pack 和 __attribute__((packed))
gcc中定義了兩個修改資料結構對齊方式的語句
1. #pragma pack()
2. __attribute__((packed))
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// calculate the offset of t in S
#define offsetof(S,t) (size_t)&(((S *)0)->t)
typedef struct _S1{
char a;
char b;
double c;
}S1;
//__attribute__((packed)) means no alignment optimization
typedef struct _S2{
char a;
char b;
double c;
}__attribute__((packed)) S2;
typedef struct _Y
{
int a;
int b;
char c;
char content[0];
} Y;
#pragma pack(push, 4)
struct a_4
{
short v1;
int v2;
};
#pragma pack(pop)
#pragma pack(push, 1)
struct a_1
{
short v1;
int v2;
};
#pragma pack(pop)
int main()
{
//the effect of __attribute__((packed)),使用了__attribute__((packed))後,沒有padding了。
printf("/nsizeof(S1)=%u, offsetof(S1,c)=%u/n", sizeof(S1),offsetof(S1,c));
printf("sizeof(S2)=%u, offsetof(S2,c)=%u/n", sizeof(S2),offsetof(S2,c));
//這個結構很有趣,這個結構Y是沒有優化的,所以編譯器給這個結構的最後增加了部分的padding
。
//所以這個結構體的大小為12,但是content的偏移卻是9
printf("sizeof(Y)=%u, offsetof(Y, content)=%u, offsetof(Y, c)=%u/n", sizeof(Y), offsetof(Y, content), offsetof(Y, c));
// one example of #pragma pack() directives
printf("sizeof(a_1)=%u /n", sizeof(struct a_1));
printf("sizeof(a_4)=%u /n", sizeof(struct a_4));
getchar();
return 0;
}